An inductive plasma processor treats workpieces with an RF plasma in a vacuum chamber and includes a coil responsive to an RF source. The coil, which can be planar or spherical or dome shaped, is driven by the RF source to generate electromagnetic fields that excite ionizable gas in the chamber to produce a plasma. Usually the coil is on or adjacent to a dielectric window that extends in a direction generally parallel to a planar horizontally extending surface of the processed workpiece. The excited plasma interacts with the workpiece in the chamber to etch the workpiece or to deposit material on it. The workpiece is typically a semiconductor wafer having a planar circular surface or a solid dielectric plate, e.g., a rectangular glass substrate used in flat panel displays, or a metal plate.
Ogle, U.S. Pat. No. 4,948,458 discloses a multi-turn spiral planar coil for achieving the above results. The spiral, which is generally of the Archimedes type, extends radially and circumferentially between its interior and exterior terminals connected to the RF source via an impedance matching network. Coils produce oscillating RF fields having magnetic and electric field components that penetrate through the dielectric window to excite electrons and ions in a portion of the plasma chamber close to the window. The spatial distribution of the magnetic field in the plasma portion close to the window is a function of the sum of individual magnetic field components produced by the current at each point of the coils. The inductive component of the electric field is produced by the time varying magnetic field, while the capacitive component of the electric field is produced by the RF voltage in the coils. The inductive electric field is azimuthal while the capacitive electric field is vertical to the workpiece. The current and voltage differ at different points because of transmission line effects of the coil at the frequency of the RF source.
For spiral designs as disclosed by and based on the Ogle '458 patent, the RF currents in the spiral coil are distributed to produce a toroidal shaped electric field resulting in a toroidal plasma close to the window, which is where power is absorbed by the gas to excite the gas to a plasma.. The toroidal shaped magnetic field is accompanied by a ring shaped electric field which generates a toroidal shaped plasma distribution. At low pressures, in the 1.0 to 10 mTorr range, diffusion of the plasma from the toroidal shaped region where plasma density is peaked tends to smear out plasma non-uniformity and increases plasma density in the chamber center just above the center of the workpiece. However, the diffusion alone generally can not sufficiently compensate plasma losses to the chamber walls and plasma density around the workpiece periphery can not be changed independently. At intermediate pressure ranges, in the 10 to 100 mTorr range, gas phase collisions of electrons, ions, and neutrals in the plasma further prevent substantial diffusion of the plasma charged particles from the toroidal region. As a result, there is a relatively high plasma density in a ring like region of the workpiece but low plasma densities in the center and peripheral workpiece portions.
These different operating conditions result in substantially large plasma flux (i.e., plasma density) variations between inside the toroid and outside the toroid, as well as at different azimuthal angles with respect to a center line of the chamber that is at right angles to the plane of the workpiece holder (i.e., chamber axis). These plasma flux variations result in a substantial standard deviation, i.e., in excess of six percent, of the plasma flux incident on the workpiece. The substantial standard deviation of the plasma flux incident on the workpiece has a tendency to cause non-uniform workpiece processing, i.e, different portions of the workpiece are etched to different extents and/or have different amounts of materials deposited on them.
Many coils have been designed to improve the uniformity of the plasma. The commonly assigned U.S. Pat. No. 5,759,280, Holland et al., issued Jun. 2, 1998, discloses a coil which, in the commercial embodiment, has a diameter of 12 inches and is operated in conjunction with a vacuum chamber having a 14.0 inch inner wall circular diameter. The coil applies magnetic and electric fields to the chamber interior via a quartz window having a 14.7 inch diameter and 0.8 inch uniform thickness. Circular semiconductor wafer workpieces are positioned on a workpiece holder about 4.7 inches below a bottom face of the window so the center of each workpiece is coincident with a center line of the coil and the chamber center line.
The coil of the '280 patent produces considerably smaller plasma flux variations across the workpiece than the coil of the '458 patent. The standard deviation of the plasma flux produced by the coil of the '280 patent on a 200 mm wafer in such a chamber operating at 5 milliTorr is a considerable improvement over the standard deviation for a coil of the '458 patent operating under the same conditions. The coil of the '280 patent causes the magnetic field to be such that the plasma density in the center of the workpiece is greater than in an intermediate part of the workpiece, which in turn exceeds the plasma density in the periphery of the workpiece. The plasma density variations in the different portions of the chamber for the coil of the '280 patent are much smaller than those of the coil of the '458 patent for the same operating conditions as produce the lower standard deviation.
Other arrangements directed to improving the uniformity of the plasma density incident on a workpiece have also concentrated on geometric principles, usually concerning coil geometry. See, e.g., U.S. Pat. Nos. 5,304,279; 5,277,751; 5,226,967; 5,368,710; 5,800,619; 5,401,350; 5,558,722, 5,795,429, 5,847,074 and 6,028,395. However, these coils have generally been designed to provide improved radial plasma flux uniformity and to a large extent have ignored azimuthal plasma flux uniformity. In addition, the fixed geometry of these coils does not permit the plasma flux distribution to be changed for different processing recipes. While we are aware that the commonly assigned co-pending U.S. application of John Holland for “Plasma Processor with Coil Responsive to Variable Amplitude RF Envelope,” Ser. No. 09/343,246, filed Jun. 30, 1999, and Gates U.S. Pat. No. 5,731,565 disclose electronic arrangements for at will controlling plasma flux uniformity for different recipes, the Holland and Gates inventions are concerned primarily with radial, rather than azimuthal, plasma flux uniformity. In the Holland invention, control of the plasma flux uniformity is achieved by controlling a variable amplitude envelope the RF excitation source applies to the coil. In the Gates invention, a switch or a capacitor shunts an interior portion of a spiral-like RF plasma excitation coil.
The frequency, i.e., reciprocal of wavelength, (typically 13.56 MHz) of the RF power source driving the coil and the lengths of the coil are such that there are significant standing wave current and voltage variations along the length of a particular winding. Voltage magnitude can change from about 1,000 volts (rms) to nearly zero volts, while the standing wave current can change nearly 50%. Hence, there are peak voltage and current somewhere along the length of each winding. However, we are aware of prior art including an RF source that drives an electrically short plasma excitation coil.
Our U.S. Pat. No. 6,164,241, entitled “Multiple Coil Antenna for Inductively-Coupled Plasma Generation Systems,” discloses another coil including two concentric electrically parallel windings each having first and second terminals, which can be considered input and output terminals of each winding. Each first terminal is connected via a first series capacitor to an output terminal of a matching network driven by an RF power source. Each second terminal is connected via a second series capacitor to a common ground terminal of the matching network and RF source. Each winding can include a single turn or multiple turns that extend circumferentially and radially in a spiral-like manner relative to a common axis of the two windings. Each winding is planar or three-dimensional (i.e., spherical or dome-shaped) or separate turns of a single winding can be stacked relative to each other to augment the amount of magnetic flux coupled by a particular winding to the plasma.
The value of the second capacitor connected between the second terminal of each winding and ground sets the locations of the voltage and current extrema (i.e., maximum and minimum) in each winding, as disclosed in Holland et al., U.S. Pat. No. 5,759,280, commonly assigned with the present invention. Controlling the value of the second capacitor of each winding controls the distribution of magnetic flux produced by the coil to the plasma and the plasma flux incident on the workpiece because the value of the capacitor determines the location of the maximum values of the RF standing wave current and voltage in each respective winding. The value of the first capacitor determines the maximum magnitude of the current and voltage standing waves in each winding. The values of the first capacitors are also adjusted to help maintain a tuned condition between the RF source and the load it drives, which is primarily the coil and the plasma load coupled to the coil. Adjusting the maximum magnitude and location of the standing wave current in each winding controls the plasma density in different radial and azimuthal regions of the chamber.
It is desirable, in certain instances, to maintain the current in one of the windings relatively constant while changing the current in the remainder of the coil. The RF current generates the magnetic field, and the time varying magnetic field in free space produces the inductive electric field, which in turn generates the plasma and induces a plasma “image” current which is the mirror image of the driving RF current. By maintaining the current in one of the windings relatively constant, the electric field produced by that winding and supplied to the plasma in the chamber remains relatively constant, despite variations in the electric field produced by the remainder of the coil and supplied to the plasma. Maintaining the electric field produced by one of the windings relatively constant while varying the electric field produced by the remainder of the coil and supplied to the plasma provides substantial control for the plasma density incident on the workpiece. Such control is particularly advantageous in connection with processing chambers operating with different recipes, which are performed without opening the vacuum chamber. Such chambers operate at different times under differing conditions. Examples of the different conditions are different processing gases, different pressures and different workpieces.
Consider a coil having first and second parallel, concentric windings respectively close to (1) the chamber periphery and (2) the chamber axis. The first and second windings respectively couple ring shaped electric fields to the peripheral portions of the chamber (close to the chamber wall) and to the chamber center. It is desirable, in certain instances, to maintain the current flowing in the outer winding substantially constant at times, while differing currents flow in the inner windings. This causes the outer winding to produce a substantially constant electric field in the chamber peripheral portions while the inner winding generates different electric fields in the chamber central region. Such a result is attained by simultaneously adjusting the overall impedance in each winding and the total power since these windings are closely coupled to both windings. Since these windings are closely coupled, the change of the overall impedance in each winding causes change in current splitting as well as power splitting between these windings. The current in each winding is changed as the impedance in any winding changes. Therefore, the current in one winding can be compensated by changing the total power in order to maintain constant current in that winding. The ability to maintain a constant electric field in the chamber peripheral portion results in an extra process control knob to maintain constant power deposition in that region and further maintain constant processing results (e.g. etch rate or deposition rate) on the peripheral portion of a workpiece. This process control is particularly useful to compensate changes due to process conditions. In other situations, particularly for other pressures, as discussed supra, it is desirable to maintain the electric field in the chamber center substantially constant at times while the amplitude of the electric field in a peripheral portion of the chamber is changed. This process control capability is particularly useful to compensate the plasma loss to chamber walls and electric coupling to the grounded portion of chamber walls.
It is accordingly an object of the present invention to provide a new and improved vacuum plasma processor and method of operating same wherein the plasma density incident on the workpiece can be controlled at will.
An additional object of the present invention is to provide a new and improved vacuum plasma processor and method of operating same wherein the plasma density incident on a workpiece has relatively high uniformity.
An additional object of the present invention to provide a new and improved vacuum plasma processor and method of operating same wherein plasma density incident on a workpiece of the processor has relatively high azimuthal uniformity.
A further object of the invention is to provide a new and improved vacuum plasma processor including a coil with plural parallel windings driven by a single RF source via a single matching network and having improved control for the electric and magnetic fields that are produced by the coil and coupled to plasma in the chamber.